1. Field of the Invention
This invention relates to digital-to-analog converters of the type employing differential switch circuits for switching the bit currents either to an output bus or to a circuit common line. More particularly, this invention relates to techniques for improving the performance of such differential switch circuits.
2. Description of the Prior Art
Digital-to-analog converters (DACs) generally have comprised a number of switches which are selectively operated by a multi-bit digital input signal to produce corresponding binarily-weighted bit currents to be summed as an analog output signal. Such converters ordinarily are formed as integrated circuits (ICs), with each switch comprising two bipolar transistors arranged as a differential switch-pair. As shown for example in U.S. Pat. No. 3,961,326 (R. B. Craven), the individual transistors of such a switch-pair typically are controlled by complementary signal sources which operate to turn one switch on while turning the other switch off. One of the two switches makes connection to a summing bus, and the other switch makes connection to a common line such as ground.
One problem with such prior art switching arrangements is that there is asymmetry in the switch turn-on and turn-off speeds. This can result in a momentary error in the DAC output until all of the switches have fully switched. This anomaly produces a glitch in the output which is code dependent and results in harmonic distortion or even non-harmonic spurs in the output spectrum.
A typical prior art switch circuit is shown in FIG. 1. The form of the output current from terminals IA, IB of this switch circuit is a function of the symmetry of the switching signals, and distortion of the output current could result from asymmetrical switching signals. However, even if the applied input signals INA, INB were perfectly symmetrical, the P-type differential switches MP1 and MP2 would introduce asymmetry in the complementary switching voltages VA, VB which control the switch transistors Q1, Q2. Such asymmetry results in transient output current distortion in any individual switch circuit. It also results in misalignment between the turn-on/turn-off times of the multiple switch-circuits employed in a DAC; that is, the timing of the separate switches of the DAC will not be synchronized properly due to the lack of symmetry with code dependency of the output current.